Data transmission protocol

ABSTRACT

A method of transmitting data from a transmitter towards a receiver can include forming a plurality of ordered data units each comprising a part of a data message. The method can also include transmitting at least some of the data units in order. The method can further include identifying a discard status in response of the data message. When a subset of the data units including the final one of the data units and at least one other of the data units has not been transmitted, the method can include indicating that the data message is to be discarded, specifying the identity of the final one of the data units, and transmitting only the final one of the data units from the subset.

This invention relates to a protocol for data transmission, for examplefor transmission of data units over a radio link. The protocol isespecially suitable for addressing issues surrounding deletion of statusprotocol data units in third-generation (e.g. 3GPP or the like) radiotelephone systems.

In the arrangement currently proposed for third-generation radiotelephone systems, for example the RLC (radio link control) ProtocolSpecification (Release 1999) data can be carried between a transmitterand a receiver in the form of protocol data units (PDUs). One form ofPDU is the status PDU (SDU). The SDU can carry information on the statusof the link, for example details of which PDUs have been received orwhich are detected as missing. Pieces making up an SDU may be carried ina number of AMD (RLC) PDUs.

SDUs may be transmitted by the network over a radio interface to an itemof user equipment (UE) such as a mobile phone, or by a UE over a radiointerface to the network.

A maximum permitted number of retransmissions (MaxDAT) may be specified,as may a maximum permitted period of time for transmission(Timer_Discard). If an SDU has not been successfully conveyed from thetransmitter to the receiver within MaxDAT retransmissions or withinTimer_Discard period of time it may be discarded.

The discarding of an SDU is performed by means of the “move receivingwindow super field” (MRW SUFI) command sent from the data transmitter tothe data receiver. The MWR SUFI command is send in STATUS PDU. The MRWSUFI command includes the following fields:

-   -   LENGTH. Specifies the number of SN_MRW fields in the MRW SUFI.    -   SN_MRW _(1−i). Provides a number i (equal to LENGTH-1) of fields        each indicating the end of a discarded SDU. SN_MRW _(i) is the        sequence number (SN) of the PDU that contains the LI (length        indicator) of the ith discarded SDU. The LI field is used to        indicate the end point of an SDU in an AMD PDU.    -   SN_MRW_(LENGTH) . Requests the receiver to discard all PDUs with        sequence number less than SN_MRW _(LENGTH) and to move its        receiving window accordingly. In addition, the receiver is to        discard the first N_(LENGTH) LIs and the corresponding data        bytes in the PDU with sequence number SN_MRW _(LENGTH).    -   N_(LENGTH) . Assists in indicating the end of the last discarded        SDU as described above. N_(LENGTH) indicates which LI in the PU        with sequence number SN_MRW_(LENGTH) corresponds to the last        discarded SDU. N_(LENGTH)=0 indicates that the last SDU ended in        the PU with sequence number SN_MRW_(LENGTH)-1 and that the first        data byte in the PU with sequence number SN_MRW_(LENGTH) is the        first data byte to be reassembled next

According to the RLC Protocol Specification (Release 1999), although anSDU may be discarded, all AMD PDUs carrying pieces of that SDU are stillsent over the radio interface. This means that valuable link resourcesare devoted to carrying data that has already been decided to bediscarded. The worst case is when the size of the SDU to be discarded isconsiderably large and the SDU discard procedure is executed in an earlyphase of transmission of the SDU. This could, for example, happen if:

-   -   a. one of the first AMD PDUs for that SDU is lost; or    -   b. if the SDU transmission timer expires when only few AMD PDUs        have been transmitted, for instance as a result of a long delay        in the radio interface.

In that situation the remainder of the AMD PDUs making up the alreadydiscarded SDU will still be transmitted, but merely waste radioresources because the transmitted data will never serve the user. Inaddition, this unnecessary data overhead may considerably slows down therate of transmission of useful data and hence degrades the quality ofservice.

When the triggering of the SDU discard procedure is based on the timerset for the SDU transmission time (using the value Timer_Discard), theprobability of the extra overhead and the consequent delays for thesubsequent SDU transmissions causing considerable losses of user dataand malfunction of the later data transmission procedure is particularlyhigh.

Once the discard of an SDU has been performed it would be desirable tominimise or at least reduce the link resources subsequently devoted tothat SDU. This could free up radio resources and increase thetransmission rate of useful data.

According to the present invention from one aspect there is provided amethod of transmitting data from a transmitter towards a receiver,comprising the steps of: forming a plurality of ordered data units eachcomprising a part of a data message; transmitting at least some of thedata units in order; identifying a discard status in respect of the datamessage; and if a subset of the data units including the final one ofthe data units and at least one other of the data units has not beentransmitted: indicating (for instance by means of a command) that thedata message is to be discarded; and transmitting only the final one ofthe data units from the said subset, including an indication of theending of the data message.

According to the present invention from another aspect there is provideda transmitter for communicating data to a receiver, the transmittedcomprising a data transmission control unit operable to perform thesteps of: forming a plurality of ordered data units each comprising apart of a data message; causing the transmission of at least some of thedata units in order; identifying a discard status in respect of the datamessage; and if a subset of the data units including the final one ofthe data units and at least one other of the data units has not beentransmitted: causing the transmission (for instance by means of acommand) of an indication that the data message is to be discarded; andcausing the transmission of only the final one of the data units fromthe said subset, including an indication of the ending of the datamessage.

Preferably, on identifying a discard status in respect of the datamessage; and if a subset of the data units including the final one ofthe data units and at least one other of the data units has not beentransmitted the data units of the subset other than the final one of thedata units are not transmitted from the transmitter towards thereceiver.

Suitably, the method comprises the step of allotting consecutivesequence numbers to each of the data units in order. Preferably themethod comprises the step of, before transmitting the final one of thedata units reallotting a sequence number to that data unit. Preferably,the sequence number reallotted to the final one of the data units is thesequence number one greater than the sequence number of the last dataunit transmitted. Alternatively, the sequence number reallotted to thefinal one of the data units may be a sequence number greater (mostpreferably two greater) than the sequence number of the last data unittransmitted. The method may comprise the steps of: allotting sequencenumbers to further data units to be transmitted after the said dataunits comprising part of the data massage; and before transmitting thosefurther data units realloting consecutive sequence numbers followingthat reallotted to the final one of the data units to the further dataunits.

The command indicating that the data message is to be discarded may betransmitted before or after the final one of the data units, subject toother demands of the protocol.

The data units are suitably protocol data units, preferably radio linkcontrol protocol data units.

The data message may be a status protocol data unit.

Preferably the method comprises specifying the identity of the final oneof the data units.

The transmitter and/or the receiver are preferably operable according tothe third-generation protocol specification or, suitably, a derivativethereof.

One of the transmitter and the receiver may be a mobile station. Theother of the transmitter and the receiver may be a network unit.

Preferably the final one of the data units in order includes dataspecifying the length of the data message.

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a radio telecommunications system;

FIG. 2 shows an illustrative structure of an SDU as carried by aplurality of PDUs; and

FIGS. 3 to 6 show alternative signaling arrangements for thetransmission of SDUs.

Like parts are indicated by like reference numerals in FIGS. 3 to 6.

FIG. 1 shows a radio telecommunications system. The system comprises aradio telecommunications network 1 including a plurality of base stationtransmission/reception units 2 which are capable of communicating byradio with user equipment 3. The user equipment could, for instance, bemobile stations such as mobile phones. By means of the network the UEscan communicate with each other or with other equipment such asland-line phone 4 or internet terminal 5 interconnected to the network1. The network includes network data transmission/reception controlunits 6 which control the transmission and reception of data to an fromthe UEs according to a predetermined protocol. Correspondingly, the UEsinclude UE data transmission/reception control units 7 which control thetransmission and reception of data to and from the network according tothe predetermined protocol.

The protocol used for the transmission of data between the UEs and thenetwork is suitably based on the RLC protocol, for example according tothe RLC Protocol Specification (Release 1999) or the like. However, asdescribed below, the present protocol includes provision for improvedhandling of discarded SDUs.

Procedures will now be described for the handling of discarded SDUs.These procedures are especially suitable for operation in a system suchas that shown in FIG. 1, particularly for transmission of data betweenthe network and the UEs (in either direction). However, the proceduresmay be implemented in other locations in the system of FIG. 1 and inother data transmission systems, for example over fixed rather thanradio links or over other wireless links such as infra-red links.

An SDU may be carried by a plurality of PDUs, with parts of the SDUbeing included in each of those PDUs. This situation is illustrated inFIG. 2. Each PDU has a sequence number. FIG. 2 illustrates a series ofRLC PDUs having sequence numbers from x to y. Each of those PDUs carriespart of a certain SDU. One of the PDUs carrying part of the SDU may bespecified by the sequence number (x+s), where s is an offset from x inthe range from 0 to (y−x). In this illustration x and y are notconsecutive numbers.

The series of PDUs is illustrated in FIG. 2 by line 10. PDU x isindicated at 11, PDU y at 12 and an arbitrary PDU (x+s) at 13.

During operation of the telecommunications system a decision may betaken to discard an SDU, for example on one of the grounds set outabove: the exceeding of a predetermined number of retransmissions or theexceeding of a predetermined time period for transmission. When thedecision is taken to discard the SDU an SDU discard procedure istriggered.

After the SDU discard procedure is triggered the MRW SUFI command (seeabove) is sent to the receiver, with the next AMD PDU to be sent to thereceiver being the one pointed to in the MRW SUFI command by its fieldSN_MRW_(LENGTH) . This will be either the AMD PDU carrying the LI fieldof the discarded SDU or, in the case where the rest of the AMD PDUcarrying the LI field of the discarded SDU is padding, the AMD PDUimmediately following it. No further data transmission actions,including sending, resending or triggering the SDU discard procedure,are executed for those AMD PDUs preceding that defined below and pointedto by the SN_MRW_(LENGTH) field of the MRW SUFI command. For those RLCPDUs, whose RLC PDU number is <MRW_(LENGTH) nothing is done. For thosewhose sequence number is >=MRW_(LENGTH) , they can be resent, if thetransceiver so requests. However, those AMD PDUs are not be removed fromthe transmitter's buffer of data to be transmitted until that MRW SUFIcommand has been acknowledged by the receiver (suitably by means of theMRW_ACK SUFI command) and the lower edge of the window of acceptableacknowledgements is updated.

The sending of the MRW SUFI command and the corresponding specifying tothe receiver AMD PDU can be done in four alternative ways, illustratedby FIGS. 3 to 6 respectively. In each of FIGS. 3 to 6, reference 20denotes the transmitting unit, reference 21 denotes the receiving unit,reference 22 denotes the command to discard an SDU and reference 23denotes generally data traffic from the transmitter to the receiver. Thedata traffic and the SDU discard command are shown in chronologicalorder, starting from the top of each figure.

In each embodiment it is assumed that the SDU is carried by a pluralityof PDUs as illustrated in FIG. 2 and that PDUs with sequence numbers xto (x+s) are sent from the transmitter to the receiver as shown at 24before the discard command 22 is received.

The first method is illustrated in FIG. 3.

In the first method, first the MRW SUFI command (25) is sent from thetransmitter to the receiver. After that the AMD PDU (26) carrying the LIfield indicating the end of the discarded SDU is sent using the SNcurrently allotted to it, i.e. the PDU whose sequence number is y. Thisdoes not affect the transmission of consequential AMD. PDUs, whichcontinues as usual (in-sequence, also noting the previous sendings,resendings and acknowledgements) from this point on. However, any PDUsin the range from (x+s) to y which would otherwise be sent need not nowbe transmitted. Once the MRW SUFI command has been acknowledged thosePDUs can be deleted from the transmitting unit's buffer. The AMD PDU 26is preferably sent immediately after the MRW SUFI command 25.

The second method is illustrated in FIG. 4.

In the second method, first the AMD PDU (27) carrying the LI fieldindicating the end of the discarded SDU is sent using the SN currentlyallotted to it. After that the MRW SUFI command (28) is sent. This doesnot affect the transmission of consequential AMD PDUs, which continuesas usual (in-sequence, also noting the previous sendings, resendings andacknowledgements) from this point on. The MRW SUFI command 28 ispreferably sent immediately after the AMD PDU 27. Again, any PDUs in therange from (x+s) to y which would otherwise be sent need not now betransmitted. Once the MRW SUFI command has been acknowledged those PDUscan be deleted from the transmitting unit's buffer.

In methods 1 and 2 there is a break in the sequence numbers of the PDUsthat are transmitted. Some sequence numbers are allotted to PDUs thatare not transmitted. Thus the sequence of sequence numbers as receivedby the receiver is not consecutive. It is conceivable that there may bea desire to avoid this situation. This is addressed by methods 3 and 4.

The third method is illustrated in FIG. 5.

In the third method, the PDUs making up the discarded SDU are again nottransmitted. The sequence numbers previously allotted to those PDUs areallotted to subsequent PDUs. Thus revised sequence numbers are generatedfor the AMD PDUs in the transmission buffer so as to utilise thesequence numbers of the non-transmitted PDUs. The AMD PDU carrying theLI field indicating the end of the discarded SDU is given the SN of thesecond non-transmitted AMD PDU. That SN value is given by VT(S)+1; whereVT(S) is the current value of the send state variable of thetransmitter, which stores the sequence number of the next PDU to betransmitted for the first time. The subsequent AMD PDUs in thetransmitter's buffer are renumbered in-sequence starting from thisvalue. Note that due, for instance, to ciphering issues the renumberedAMD PDUs will not have been sent yet.

The MRW SUFI command (30) and the AMD PDU (29) whose the LI fieldindicates the end of the discarded SDU are sent from the transmitter tothe receiver. In FIG. 5 the MRW SUFI command 30 is shown as being sentfirst, but since the AMD PDU 29 has been given the SN corresponding tothe second non-transmitted AMD PDU the MRW SUFI command 30 and the AMDPDU 29 may be sent in either order. This is because if the RLC PDU SNwere (x+s)+1 then the receiver RLC would forward this incorrect SDU tothe PDCP layer, because no MRW SUFI command regarding this SDU wouldhave been received by then.

The fourth method is illustrated in FIG. 6.

In the fourth method, the PDUs making up the discarded SDU are again nottransmitted. The sequence numbers previously allotted to those PDUs areallotted to subsequent PDUs. Thus revised sequence numbers are generatedfor the AMD PDUs in the transmission buffer so as to utilise thesequence numbers of the non-transmitted PDUs. The AMD PDU carrying theLI field indicating the end of the discarded SDU is given the SN of thefirst non-transmitted AMD PDU. That SN is given by the current value ofthe state variable VT(S). The subsequent AMD PDUs in the transmitter'sbuffer are renumbered in-sequence starting from this value.

The MRW SUFI command (31) and the AMD PDU (32) whose the LI fieldindicates the end of the discarded SDU are sent from the transmitter tothe receiver. In FIG. 6, since the AMD PDU 32 has been given the SNcorresponding to the first non-transmitted AMD PDU the MRW SUFI command31 must be sent before the AMD PDU 32.

The method of FIG. 6 can not be considered as reliable as that of FIG.5. In the method of FIG. 6 an error situation may occur if the MRW SUFIcommand is lost in the radio interface. Then if the following AMD PDUcontaining the LI field of the discarded SDU is sent before theacknowledgement to the MRW SUFI command has been received, the receivermay consider the SDU to be complete and deliver it to PDCP forsubsequent processing. This kind of error could be prevented by blockingthe transmission of all new AMD PDUs in the transmission buffer untilthe MRW SUFI command has been acknowledged. Alternatively it may beconvenient to block the transmission of all AMD PDUs (and thus savefurther radio resources) because, according to the recent RLCspecification, executing the SDU discard procedure for a certain SDUcauses preceding unacknowledged SDUs to be discarded as well.

In all of these cases the lower edge of the window of acceptableacknowledgements—i.e. the value of the state variable VT(A) at thetransmitter, indicating the sequence number of the next in-sequence PDUexpected to be acknowledged—is updated only after receiving theacknowledgement to the MRW SUFI command. This is similar to thesituation as currently defined in the recent RLC specification. This waythe SDU discard procedure and the sending of the MRW SUFI command doesnot implicitly acknowledge AMD PDUs, which could easily lead to the“run-away” of the sending window and finally to non-synchronized datatransmission.

Following the procedures illustrated in FIGS. 3 to 6, and particularlysubsequent to the sending of the first MRW SUFI command as shown, theexecution of the SDU discard procedure continues as described in the RLCspecification.

The procedures set out with reference to FIGS. 3 to 6 offer significantsavings in radio resources over prior methods, especially in situationswhere discard of SDUs is frequent, and even more especially when discardof long SDUs may be executed shortly after the SDUs have started to betransmitted. Such circumstances may arise when there are longtransmission delays and/or high numbers of lost AMD PDUs. Furthermore,the approaches described above allow the possibility of improvingquality of service, by making SDU delays shorter and lowering the numberof lost SDUs; making the protocol less sensitive to errors due to lostor delayed AMD PDUs; and improving the protocol's ability to recoverfrom error situations.

When PDUs of a data message (SDU) have been discarded, the transmissionof that data message may be ended by the transmission of a final dataunit of the set of data units representing that message. That final dataunit can include a length indicator (LI) pointing to the start of thetransmitted data of the message, whereby it can be determined at thereceiver that transmission of the message has ended.

The applicant draws attention to the fact that the present invention mayinclude any feature or combination of features disclosed herein eitherimplicitly or explicitly or any generalisation thereof, withoutlimitation to the scope of any of the present claims. In view of theforegoing description it will be evident to a person skilled in the artthat various modifications may be made within the scope of theinvention.

1. A method, comprising: forming a plurality of ordered data units eachcomprising a part of a data message; allotting sequence numbers to eachof the ordered data units; transmitting at least some of the data unitsin order; identifying a discard status in respect of the data message;and when a subset of the ordered data units including the final one ofthe ordered data units in the order and at least one other of the dataunits has not been transmitted indicating that the data message is to bediscarded, and re-allotting a sequence number two greater than thesequence number of the last data unit transmitted to the final one ofthe ordered data units; and transmitting the final one of the ordereddata units from the subset, including an indication of the ending of thedata message.
 2. The method of claim 1, wherein the including theindication of the ending of the data message comprises including anindication specifying the location of the start of the data message. 3.The method of claim 1, wherein on the identifying the discard status inrespect of the data message; and when the subset of the data unitsincluding the final one of the ordered data units and at least one otherof the data units has not been transmitted, the data units of the subsetother than the final one of the ordered data units are not transmittedfrom the transmitter towards the receiver.
 4. The method of claim 1,further comprising: allotting sequence numbers to further data units tobe transmitted after the data units comprising part of the data message;and before transmitting those further data units reallotting consecutivesequence numbers following that reallotted to the final one of theordered data units to the further data units.
 5. The method of claim 1,wherein the indicating that the data message is to be discardedcomprises transmitting an indication after transmitting the final one ofthe ordered data units.
 6. The method of claim 1, wherein thetransmitting the final one of the ordered data units is performed aftertransmitting an indication that the data message is to be discarded. 7.The method of claim 1, wherein the transmitting the at least one of thedata units comprises transmitting protocol data units.
 8. The method ofclaim 7, wherein the transmitting the protocol data units comprisestransmitting radio link control protocol data units.
 9. The method ofclaim 1, wherein the identifying the discard status in respect of thedata message comprises identifying the discard status in respect of astatus protocol data unit.
 10. The method of claim 1, wherein one of thetransmitter and the receiver is a mobile station.
 11. The method ofclaim 10, wherein the other of the transmitter and the receiver is anetwork unit.
 12. The method of claim 1, wherein the transmitting the atleast some of the data units and the transmitting the final one of theordered data units from the subset comprise transmitting by a wirelesslink.
 13. The method of claim 12, wherein the transmitting the at leastsome of the data units and the transmitting the final one of the ordereddata units from the subset comprise transmitting by radio.
 14. Themethod of claim 1, wherein the transmitting the final one of the ordereddata units in order includes transmitting data specifying the length ofthe data message.
 15. The method of claim 1, wherein the transmittingthe at least some of the data units comprises transmitting in anacknowledged mode.
 16. The method of claim 1, wherein the transmittingthe at least some of the data units comprises transmitting in anunacknowledged mode.
 17. A method, comprising: forming a plurality ofordered data units each comprising a part of a data message; allottingsequence numbers to each of the ordered data units; transmitting atleast some of the data units in order; identifying a discard status inrespect of the data message; and when a subset of the ordered data unitsincluding the final one of the ordered data units in the order and atleast one other of the data units has not been transmitted indicatingthat the data message is to be discarded, reallotting a sequence numberone greater than the sequence number of the last data unit transmittedto the final one of the ordered data units; and transmitting the finalone of the ordered data units from the subset, including an indicationof the ending of the data message.
 18. An apparatus, comprising: aprocessor configured to form a plurality of ordered units eachcomprising a part of a data message; and a transmitter configured totransmit at least some of the data units in order and allocate sequencenumbers to each of the ordered data units, wherein the processor isconfigured to identify a discard status in respect of the data message,wherein, when a subset of the data units including the final one of theordered data units in order and at least one other of the data units hasnot been transmitted the transmitter is configured to indicate that thedata message is to be discarded, and the transmitter is configured tore-allot a sequence number two greater than the sequence number of thelast data unit transmitted to the final one of the ordered data unitsand to transmit the final one of the ordered data units from the subset,including an indication of the ending of the data message.
 19. Theapparatus of claim 18, wherein the transmitter is configured to reallotthe sequence number by reallotting to the final one of the ordered dataunits the sequence number one greater than the sequence number of thelast data unit transmitted.
 20. The apparatus of claim 18, wherein thetransmitter is configured to allot sequence numbers to further dataunits to be transmitted after the data units comprising part of the datamessage and, before transmitting those further data units, thetransmitter is configured to reallot consecutive sequence numbersfollowing that reallotted to the final one of the ordered data units tothe further data units.
 21. The apparatus of claim 18, wherein thetransmitter is configured to transmit the final one of the ordered dataunits by a wireless link.
 22. The apparatus of claim 21, wherein thetransmitter is configured to transmit the final one of the ordered dataunits by radio.
 23. The apparatus of claim 18, wherein the transmitteris configured to transmit, with the final one of the ordered data unitsin order, data specifying the length of the data message.
 24. Theapparatus of claim 18, wherein the indication of the ending of the datamessage comprises an indication specifying the location of the start ofthe data message.
 25. The apparatus of claim 18, wherein on theidentifying the discard status in respect of the data message; and whenthe subset of the data units including the final one of the ordered dataunits and at least one other of the data units has not been transmitted,the transmitter is configured not to transmit the data units of thesubset other than the final one of the ordered data units.
 26. Anapparatus, comprising: a data transmission controller configured to forma plurality of ordered data units each comprising a part of a datamessage, allot sequence numbers to each of the ordered data units; causethe transmission of at least some of the data units in order; identify adiscard status in respect of the data message; and when a subset of thedata units including the final one of the ordered data units in orderand at least one other of the data units has not been transmitted, causethe transmission of an indication that the data message is to bediscarded, re-allot a sequence number two greater than the sequencenumber of the last data unit transmitted to the final one of the ordereddata units and cause the transmission of the final one of the ordereddata units from the subset, including an indication of the ending of thedata message.
 27. An apparatus, comprising: formation means for forminga plurality of ordered units each comprising a part of a data message;allotting means for allotting sequence numbers to each of the ordereddata units; transmission means for transmitting at least some of thedata units in order; and identification means for identifying a discardstatus in respect of the data message, wherein, when a subset of thedata units including the final one of the ordered data units in orderand at least one other of the data units has not been transmitted thetransmission means is configured to indicate that the data message is tobe discarded, wherein the apparatus further comprises: re-allottingmeans for re-alloting a sequence number two greater than the sequencenumber of the last data unit transmitted to the final one of the ordereddata units and the transmission means is configured to transmit thefinal one of the ordered data units from the subset, including anindication of the ending of the data message.
 28. An apparatus,comprising: a processor configured to form a plurality of ordered unitseach comprising a part of a data message; and a transmitter configuredto transmit at least some of the data units in order and allocatesequence numbers to each of the ordered data units, wherein theprocessor is configured to identify a discard status in respect of thedata message, wherein, when a subset of the data units including thefinal one of the ordered data units in order and at least one other ofthe data units has not been transmitted the transmitter is configured toindicate that the data message is to be discarded, and the transmitteris configured to re-allot a sequence number one greater than thesequence number of the last data unit transmitted to the final one ofthe ordered data units and to transmit the final one of the ordered dataunits from the subset, including an indication of the ending of the datamessage.
 29. An apparatus, comprising: a data transmission controllerconfigured to form a plurality of ordered data units each comprising apart of a data message, allot sequence numbers to each of the ordereddata units; cause the transmission of at least some of the data units inorder; identify a discard status in respect of the data message; andwhen a subset of the data units including the final one of the ordereddata units in order and at least one other of the data units has notbeen transmitted, cause the transmission of an indication that the datamessage is to be discarded, re-allot a sequence number one greater thanthe sequence number of the last data unit transmitted to the final oneof the ordered data units and cause the transmission of the final one ofthe ordered data units from the subset, including an indication of theending of the data message.
 30. An apparatus, comprising: formationmeans for forming a plurality of ordered units each comprising a part ofa data message; allotting means for allotting sequence numbers to eachof the ordered data units; transmission means for transmitting at leastsome of the data units in order; and identification means foridentifying a discard status in respect of the data message, wherein,when a subset of the data units including the final one of the ordereddata units in order and at least one other of the data units has notbeen transmitted the transmission means is configured to indicate thatthe data message is to be discarded, wherein the apparatus furthercomprises: re-allotting means for re-alloting a sequence number onegreater than the sequence number of the last data unit transmitted tothe final one of the ordered data units and the transmission means isconfigured to transmit the final one of the ordered data units from thesubset, including an indication of the ending of the data message.